In order to avoid faulty products or in order to remove the latter as rapidly as possible after its detection, printed products are controlled in various ways during their further processing, which for example can involve the stages of folding, cutting, collecting, binding and/or stitching or stapling, where the printed products are advantageously monitored without their continuous conveying effected by a random sensor arrangement being impaired. There are sensor arrangements operating in a contactless manner (light barriers, image recording and processing methods, induction measurements, etc.) and sensor arrangements enabling the printed product to be scanned (mechanical probes, thickness measurements, etc.).
In many cases, where both control methods with contactless sensor arrangements and control methods in volume contact can be implemented, preference is given to the contactless method, although in many cases it is more complicated. This more particularly applies in the case of controls of sensitive products conveyed at high speeds, because as a result of the contactless control it is possible to exclude any marking on or risk of damage to the sensitive products being conveyed.
An example of such a contactless control of continuously conveyed printed products is the control of stapling by a contactless induction measurement of the stapling point, which in the presence of a metal staple fitted at the intended stapling point in the intended manner supplies a different measurement result as compared with when the staple is omitted. This stapling control is contactless and there is no risk that the printed products, even if very rapidly conveyed, could in any way be undesirably marked or damaged. However, this contactless control is not only complicated, but also in many cases unreliable, as will be shown hereinafter.
Folded sheets are for example collected on a continuously moving, saddle-shaped support, in such a way that the folded edges of the collected or collated sheets are superimposed over the saddle line (or over a narrow saddle face) of the support and the two folded parts are located on both sides of the saddle line. The finished, brochure-like group of folded sheets is then guided with the support through at least one stitching or stapling station, where the folded sheets are stapled together by means of at least one wire staple. Conventionally, the staples (a piece of wire bent at both sides) is driven from the outside of the folded edge through the group of sheets and closed on the inside of the folded edge, in that the bent wire ends are bent against one another against the saddle face of the support or by correspondingly inserted bending tools. As a result of the stapling process, a stapled printed product is obtained from a plurality of brochure-like, folded sheets arranged on the support and the staple or staples are positioned over the support saddle line. For checking the stapling of the folded sheets the support with the stapled printed product is guided into the vicinity of a sensor, which scans in contactless manner by an induction measurement, the staple part located on the outside of the folded edge.
As stated hereinbefore, the control of the stapling by an induction measurement in the vicinity of the stapling point following the stapling process is complicated and leads to very varied difficulties. On the one hand the measurement result is sensitive as a function of the precise distance between the staple and the measuring head, this distance not only being dependent on the thickness (number and thickness of the collated sheets) of the group of sheets to be stapled together, but in particular on the precise shape of the staple in the stapled product. The distance between the measuring head and the staple part to be detected is admittedly adjusted for particular products to be stapled, but not for a specific staple shape, because the shape of a staple can vary as a function of variations in the bending process for the staples between individual products even in the case where the products being stapled have the same thickness. On the other hand the inductive measurement can be very easily disturbed by other, metallic parts present in the overall arrangement (e.g. the support or parts thereof) and in particular by other staples which are not to be controlled and with which for example subgroups of collected sheets have been stapled together beforehand and where the position of those staples on the length of the folded edge is not in certain circumstances accurately defined.
Other examples of the disadvantages of using contactless control methods, particularly with inductive measuring methods, also exist in other printed product processing fields.